1. Field of the Invention
This invention relates to a novel cyanine compound, more particularly a cyanine compound having a specific structure suitable for use as an optical element, especially as a light absorber in an optical filter of an image display device or an optical recording agent used in a laser optical recording material.
2. Description of the Related Art
Compounds having a high absorption in a wavelength range of 500 to 700 nm, especially those having an absorption peak wavelength (λmax) between 550 nm and 620 nm, are used as an optical element in a recording layer of optical recording media including DVD-Rs or in an optical filter of image displays including liquid crystal displays (LCDs), plasma display panels (PDPs), electroluminescent displays (ELDs), cathode ray tube displays (CRTs), fluorescent display tubes, and field emission displays (FEDs).
High-sensitivity cyanine compounds having an indole ring have been studied as a promising optical element for these applications. Capable of coping with the increasing recording speed, the indole ring-containing cyanine compounds are reported as a recording element of optical recording media represented by DVD-Rs in not a few publications, for example, JP-A-10-278426, JP-A-11-227331, JP-A-11-277904, and JP-A-2002-52829. The first three publications disclose indole ring-containing cyanine compounds which may have an aralkyl group at the 3-position of the indole skeleton. While these cyanine compounds are relevant to the compounds of the present invention, the publications are silent on the method of introducing an aralkyl group into the cyanine compounds and the effects brought about by the introduction of an aralkyl group. In addition, these and other known cyanine compounds are unsatisfactory in thermal decomposition characteristics. Optical elements for applications to high speed recording are obviously required to have low decomposition temperatures. The compounds disclosed in the above cited four publications are unsatisfactory from that viewpoint.